Surge arresters are typically connected between the line and ground conductors of an electrical power system and are used to protect electrical equipment connected to those conductors from overvoltage surges appearing thereon. Such surges may be caused, for example, by lightning or by switching.
Surge arresters fall within one of three general categories depending upon their size and/or usage. Station class surge erresters are used in electrical substations to protect electrical equipment connected to power lines having a voltage of, for example, 100,000 volts (100 kV) or more. Distribution class surge arresters, typically placed on the poles supporting overhead power lines, are used to protect the primary side of transformers which step down the voltage on the overhead lines to levels usable in residential or commercial buildings. Thus, distribution class surge arresters protect equipment connected to power lines carrying intermediate voltages which fall in the range of from about 1000 volts up to about 50 kV. Secondary surge arresters are used to protect electrical equipment connected to power lines carrying voltages 1000 volts and below. Such secondary surge arresters are typically applied anywhere from the secondary terminals of the distribution transformer to the service entrance panel within commercial and residential buildings.
Surge arresters, having one or more non-linear voltage dependent resistive elements, such as metal oxide varistor (MOV) arrester elements, are generally referred to as gapless surge arresters. In normal steady-state operation, an MOV arrester element has a high resistance while drawing only minimal current such that the surge arrester acts essentially as an open circuit in the system. However, when an overvoltage surge occurs on the electrical lines protected by the surge arrester, the resistance of the MOV arrester element lowers to allow the surge arrester to act essentially as a short circuit. Accordingly, the surge arrester passes the surge current to ground before the coincident voltage has a chance to damage equipment connected to the protected electrical lines.
The current drawn by gapless surge arresters, even the small current drawn during steady state operation, produces I.sup.2 R losses in the form of heat. The casing of the surge arrester must be able to dissipate this heat to the surrounding environment. If the heat from I.sup.2 R losses exceeds the ability of the casing to dissipate it, thermal runaway of the gapless surge arrester can occur. Thermal runaway results in destruction of the MOV arrester element and, thus, arcing. Because an arc burns at an extremely high temperature, e.g. 6000-7000 degrees Kelvin, everything within the path of the arc is vaporized. As a result, gas pressure within the surge arrester casing builds up. If the gas pressure is not limited, the casing of the surge arrester will fragment violently presenting a safety risk to adjacent equipment and/or persons in the vicinity.
The ability of the casing to dissipate the heat from I.sup.2 R losses is affected by the ambient temperature; that is, as the ambient temperature increases, either the casing is able to dissipate less heat or, conversely, the MOV arrester elements will run hotter to produce a temperature differential which is sufficient to dissipate the heat. The gapless surge arrester must be designed for the highest expected ambient temperature so that the casing can adequately dissipate the heat resulting from such I.sup.2 R losses.
Arcing within a gapless surge arrester can also result from other causes. For example, a breakdown of the MOV arrester element insulation coating or a thermal shock to an MOV arrester element caused, for example, by an abnormally high current spike due to a nearby lightning strike, can produce holes or gaps in the MOV arrester element, resulting in its failure. Current then arcs through these holes or gaps leading to vaporization of the failed MOV arrester element and gas pressure buildup as previously described. The resulting violent fragmentation of the arrester casing can injure anyone in the vicinity of the surge arrester. This safety risk increases if these holes or gaps appear in the MOV arrester element immediately before an external fuse is blown or a circuit breaker opens. In this situation, arcing may occur when the user reestablishes the circuit to the surge arrester by closing the circuit breaker or changing the fuse, at which time arcing and the buildup of gas pressure within the surge arrester casing will occur while the user is still in the vicinity of the surge arrester.
To reduce this safety risk, surge arresters have been designed to limit gas pressure buildup by venting such pressure through a rupturable diaphragm, a releasable end cap, or other vent before the arrester explodes. These designs sometimes have an isolating feature, for example, a ground lead disconnector which isolates the surge arrester from the system by mechanically disrupting an electrical connection. Such designs are disclosed in Schmalz, et al., U.S. Pat. No. 3,803,524 and Talbot, et al., U.S. Pat. No. 4,649,457. These designs do not, however, extinguish the arc during a failure of the surge arrester, and thus also present a significant safety risk in that hot pressurized gas is released outside of the surge protector casing making such surge arresters particularly dangerous for use as secondary surge arresters.
Woodworth, et al., U.S. Pat. No. 4,930,039, discloses a station class surge arrester having a vented lining surrounding varistor elements connected in series. Upon failure, internal arcing vaporizes the varistor elements producing ionized gas which escapes through the vented lining. The arc follows the escaping gas outside of the lining thereby preventing further disintegration of the varistor elements and further buildup of gas pressure within the lining such that the outer casing of the surge arrester does not fragment violently. An isolation fuse element may be included in series with the varistor elements in order to eliminate the need for a ground lead disconnector. An isolation fuse, particularly at the voltage levels experienced by station class surge arresters and by distribution class surge arresters, does not of itself extinguish arcing.
Sweetana, Jr., et al., U.S. Pat. No. 4,223,366, discloses a station class gapless surge arrester having a stack of zinc oxide disks disposed within a porcelain casing. A space between the outside of the disks and the inner wall of the porcelain casing is filled with sand or other thermally conductive media in order to reduce the thermal resistance between the zinc oxide disks and the casing so as to more efficiently transfer heat from the zinc oxide disks to the porcelain casing during normal operation. This configuration allows the surge arrester to operate in higher ambient temperatures or at higher steady-state voltages before entering a thermal runaway condition. The sand changes state to absorb a limited amount of fault energy in the event of a failure of the zinc oxide disks. The sand, however, is insufficient to extinguish arcing within the surge arrester structure and, therefore, does not prevent explosive fragmentation of the surge arrester.
Another known prior art surge arrester has a stack of donut shaped MOVs enclosed in an outer cylindrical casing. Rupturable diaphragms cover the casing ends. Sand fills a gap between the outside of the MOVs and the inside of the casing while air fills the inner circular space of the donut shaped MOVs. In the event of a failure of one of the MOVs, any arcing, which prefers the air filled space to the sand filled space, is directed through the inner circular space of the donut shaped MOVs. Thus, the arc is prevented from thermally shocking the outer casing and causing it to rupture. Gas produced by the arc, however, increases the pressure within the casing which forces the diaphragms to expand and rupture thereby releasing hot gas outside of the casing.
It is further known that a current limiting fuse can be externally placed in series with a surge arrester to protect against violent failures of the surge arrester. This arrangement, however, substantially increases the cost of the surge arrester in that current limiting fuses can cost as much, if not more, than standard surge arresters available in the marketplace. Also, such a configuration is difficult to implement in normal residential and commercial buildings where space is limited. Still further, failure of such a surge arrester configuration is not easily detectable because this surge arrester configuration does not fragment, rupture a diaphragm or provide any other visual physical sign indicating the failure thereof.
Another known surge arrester includes a plurality of MOVs which are mounted in parallel on a circuit board and which are sealed within a casing filled with sand. Each of the MOVs is connected in series with a fuse wire between a line terminal and a neutral or ground terminal which terminals are coupled to electrical lines of an electrical power system. The parallel-connected MOVs operate as a short circuit between the line and ground terminals when an overvoltage surge appears on the electrical lines. Furthermore, the fuse wires and the sand operate as current limiting fuses during a failure of any of the MOVs to prevent buildup of gases within the casing and subsequent fragmentation thereof.
This surge arrester also includes either neon lights or light emitting diodes (LEDs) which, along with diodes and dropping resistors, are coupled across the line and the ground terminals to indicate a failure of one or more of the MOVs. This indicating feature, however, substantially increases the cost of the surge arrester due to the extra circuitry and manufacturing steps required therefor. Furthermore, both the LEDs and the neon bulbs violate the integrity of the casing seal which may cause leaks within the casing and lead to premature failure of one or more of the MOVs. Still further, the casing of this surge arrester must be made larger than would be otherwise necessary in order to dissipate the additional heat generated by the I.sup.2 R losses within the neon lights, the LEDs and the dropping resistors. Moreover, because a lighted neon bulb or an emitting LED indicates proper operation of this surge arrester, a burned out bulb or a failed LED falsely indicates failure of this surge arrester which results in the premature replacement thereof.